Silicon-based photonics has grown to include dielectric waveguides, as well as silicon waveguides. A dielectric waveguide carries light in a central core made of an amorphous dielectric material, such as silicon nitride or silicon dioxide, and is typically formed on a silicon substrate. A silicon waveguide carries light in a core made of pure single-crystal silicon, which is typically formed from the active region of a silicon-on-insulator (SOI) wafer.
Though developed separately, some properties of these two waveguide technologies are complementary. For example, relative to silicon waveguides, dielectric waveguides can be optically coupled with optical fibers with less loss, exhibit lower propagation loss, and are generally less sensitive to perturbations. In addition, dielectric waveguides can carry light signals over a wide spectral bandwidth that includes the ultraviolet, visible, and near-infrared wavelength ranges. Further, dielectric waveguides can be fabricated such that they exhibit extremely low optical propagation loss (i.e., “ultra-low-loss waveguides”). Silicon waveguides, on the other hand, offer tighter bend radii and can be integrated with modulators, detectors, and CMOS electronics.
Historically, the choice of waveguide technology has been based upon the specific needs of the application in which it was to be used. In applications more sensitive to optical loss or that required large wavelength bandwidth, for example, dielectric waveguides have typically been selected. In applications where chip real-estate was a premium, or integration with electronic or active photonic devices was desired, silicon waveguides have been preferred.
Recently, complementary properties of these waveguide technologies have been combined in a single chip by forming dielectric waveguides on substrates on which silicon waveguides, silicon electronics, and compound semiconductor devices have previously been fabricated. Unfortunately, integration of these disparate waveguide technologies, using methods of the prior art, requires compromises in waveguide and/or device performance.
As-deposited dielectric-waveguide materials are characterized by high hydrogen concentration, which gives rise to optical propagation loss. As a result, after a dielectric waveguide is defined, it must be annealed at high-temperature (typically above 900° C.) to reduce hydrogen concentration to achieve low optical loss. Electronic and photonic devices are extremely sensitive to temperature, however. For example, exposure to high temperature can damage the doped regions of electrically active devices (used to form p-n junctions, electrical connections, etc.). In addition, electrically active devices are normally interconnected via metal traces, which cannot be subjected to high temperatures. Further, compound semiconductor epitaxial layers are also damaged by the high temperature processing and annealing required to produce low-loss dielectric waveguides. These factors preclude the use of the high-temperature annealing necessary to produce low-loss dielectric waveguides formed on a substrate that contains active electrical devices. As a result, prior-art dielectric/silicon waveguide integration platforms have been limited to low temperature processes in which dielectric waveguides are not annealed.
To further complicate matters, it is difficult to provide a lower cladding for the integrated dielectric waveguide that has thickness sufficient to optically isolate the dielectric waveguides from the silicon-waveguide substrate. The thickness of this lower cladding is limited to the thickness of the buried oxide (BOX) layer of the underlying SOI substrate plus the thickness of the silicon dioxide layer formed between the silicon waveguide and the dielectric waveguide core. Unfortunately, thick BOX layers are difficult to fabricate and are, therefore, costly. As a result, prior-art dielectric waveguides integrated with silicon waveguides normally exhibit significant optical loss due to light leakage into the underlying substrate.
A waveguide integration platform capable of integrating low-loss dielectric waveguides with silicon and/or compound semiconductor waveguides would be a significant advance in the state of the art.